Multiple function spectrophotometer having an automatically stabilized chopper disc

ABSTRACT

A spectrophotometer of the two-beam type employing two adjustable monochromators and employing a rotating chopper disc to provide time-sharing operation, such as split-beam operation, dual wavelength operation, or other desired modes of operation. The chopper disc motor is stabilized by a fiber optic feedback system and cooperating electronic system utilizing light beams reflected by the underside of the chopper disc and photo diodes responding to these reflections. The main optical system includes an adjustable lens element coupled to one of the monochromators to compensate for variations in refractive index of the lens element with wavelength. The electronic readout system is gated by the chopper disc stabilizing system to maintain synchronism of the readout signals with the two monochromatic excitation beams.

nited States Patent 1191 Lowy et al.

[ Ian. 22, I974 Prim'ary Examiner-Ronald L. Wibert Assistant ExaminerV.P. McGraw Attorney, Agent, or Firml-Ierman L. Gordon [75] Inventors:George W. Lowy; Joseph [57] ABSTRACT Conlon, both of Silver Spring,Byron Marcus, Columbia, n f Md A spectrophotometer of the two-beam typeemploying two adjustable monochromators and employing a ro- [73]Asslgnee: Baxer Labm'amnes Morton tating chopper disc to providetime-sharing operation,

Grove such as split-beam operation, dual wavelength opera- 22 Filed; 11972 tion, or other desired modes of operation. The chopper disc motoris stabilized by a fiber optic feedback [21] Appl. No.: 315,875 systemand cooperating electronic system utilizing light beams reflected by theunderside of the chopper 52 US. (:1 356/93, 356/51, 356/94, disc andPhoto diodes responding to these reflections- 356/97 35 /100 250/233 Themain optical system includes an adjustable lens 51 Im. on Glj 3/42, GOlj3/12 element coupled to one of the monochromators o 53 Field f Search";/93 94 95 97 99 100 compensate for variations in refractive index of the35 01 51; 250 233; 350/274 lens element with wavelength. The electronicreadout system is gated by the chopper disc stabilizing system 5References Cited to maintain synchronism of the readout signals withUNITED STATES PATENTS the two monochromatic excitation beams.

3,712,733 1/1973 Yamamoto 356/93 19 Claims, 5 a ng igures DCJUPPLY L 5a? 84 ew fi DC n;

Aware/z q0 85 HIGH 62 7 76 firmer/V5 REE VOLT/965$ v 3.5 fthtFACEJ g] 834 i 39 43 Pam-0 D ans rK.8

a 4 3 Z T 4f 4! 4/7 .93 66 g V5 '(STEHDY) AMPLE/7am 6mm .5 7 6 148 7/ 734 Pliers 0100: H 7 50 /0! it 55' '1 -7o v 72 P FE 2 9'0 1 p Pi Cu/Z Er vi fllffi'flM/ FF. I E 'k 0 72: 01.7 905 7 V j PB 4 H J] 6% R(T'90YReta/205k l MULTIPLE FUNCTION SIECTROPHOTOMETER HAVING AN AUTOMATICALLYSTABILIZED CHOPPER DISC of operation, such as in the split beam mode,the dual wavelength mode, the dual wavelength scanning mode, and variousother modes, and which provides wide flexibility of operation, expandedsample-handling capability, and improved optical performance.

A further object of the invention is to provide an improyedspectrophotometer of the type employing two monochromatic excitationbeams and an accompanying system which can be adjusted to provide a widerange of different modes of operation, such as a splitbeam mode whereina single monochromator is used to scan either the entire usablewavelength range or a rel-' atively small increment of the range, andwherein the working beam is time-shared through a reference cell and asample cell, a dual wavelength mode wherein one monochromator is set ata reference wavelength (such as an isosbestic point) and the othermonochromator is set at a different wavelength (such as a nearbyabsorption peak) and the reference and sample beams are time-sharedthrough a single cell, a dual-wavelength scanning mode wherein onemonochromator is set at a reference wavelength and the othermonochromator scans over a wavelength increment, with the reference andsample beams being time-shared through a single cell, a derivative modewherein the two monochromators are locked together at slightly differentwavelengths, the two beams being time-shared through a single cell and aderivative spectrum being obtained by scanning over a pre-selectedwavelength range, a rapid kinetics mode wherein the optical chopperoperates at relatively high speed, or in a single beam mode, thespectrophotometer having a high degree of reliability, being stable inperformance, and'having wavelengthcompensating lens adjusting means.

A still further object of the invention is to provide an improvedspectrophotometer as'above described having dual radiation sourcesproviding different types of emission, with means to utilize eitherradiation source, whereby to provide a widened range of excitationwavelengths.

Further objects and advantagesof the invention will become apparent fromthe following description and claims, and from the accompanyingdrawings, wherein:

FIG. 1 is a diagram schematically showing the main components of animproved spectrophotometer constructed in accordance with the presentinvention.

FIG. 2 is an enlarged plan view of the optical chopper disc employed inthe spectrophotometer of FIG. I, said view being taken substantially onthe line 22 of FIG.

FIG. 3 is a schematic block diagram showing the main electricalcomponents of the spectrophotometer of FIG. 1 and showing schematicallythe optical compon ents of the chopper disc stabilizing system employedin the spectrophotometer.v

FIG. 4 is a vertical cross-sectional view taken through the focussinglens housing and the monochromatic grating adjusting assembly formingpart of the spectrophotometer of FIG. 1, showing the mechanical couplingbetween the grating and the movable lens element for providingwavelengthcompensation.

FIG. 5 is a cross'sectional'view taken substantially on the line 55 ofFIG. 4.

Referring to the drawings, and more particularly to FIG. I, 11 generallydesignates a typical multiplefunction spectrophotometer constructed inaccordance with the present inventiomThe spectrophotometer 11 comprisesalternative radiation sources 12 and 13 which may be employed to provideexcitation radiation. Source 12 comprises a tungsten lamp or othersuitable source of visible light, and source 13 comprises a deuteriumlamp or other suitable source of radiation extending into theultra-violet range, whereby the two sources afford access to a widerange of excitation wavelengths, namely, from approximately 200 to 1200nm. A pivoted mirror 14 is employed to select the desired source and tofocus radiation therefrom through a suitable double-aperture plate- 16and through a lens 15 onto an entrance slit 190. Plate 16 has theapertures 17 and 18 for providing two radiation beams 19 and 20. Thebeam 20 may at times be cut off, by the provision of a pivoted gate 21which blocks aperture 18 when it is swing to the dotted view positionthereof shown in FIG. 1. In the dual wavelength mode shown in full-lineview in FIG. 1, the gate 21 allows radiation to pass through bothapertures 17 and 18.

The beams 19 and 20 passing through the lens 15 are directed to a firstinclined stationary mirror 21 and are reflected therefrom towardrespective pivoted reflectance gratings 22 and 23 which operate asmonochromators, providing the selection of two respective excitationbeams 24 and 25 of different wavelengths, Said excitation beams arereflected from a second inclined stationary mirror 26 through a doubletfocussing lens system including a fixed lens 114 and an adjustable lens27toward a 45 inclined reflective chopper disc 28 having a parallelstationary mirror 29 disposed behind its plane and located to receivebeam 25 through the notches of the chopper disc, as diagrammaticallyillustrated in FIG. 1, to provide resultant time-shared beams 24,25directed toward a stationary sample cell 30. Said time-shared beams,after travelling through sample cell 30 and its contents, impinge on thesensing portion of a photomultiplier tube 31. 7

Another parallel stationary mirror 32 is provided in the path of thatportion of beam 24.passing through the notches of chopper disc 28, saidmirror 32 being located so as to direct said passing beam portion towarda stationary reference cell 33 located adjacent the sensing portion ofphotomultiplier tube 31, as shown, to allow said passing beam portion toimpinge on said sensing portion after passingthrough reference cell 33,whereby to provide a time-shared split-beam mode of operation. A pivotedgate 34' is provided between mirror 32 and reference cell 33, which, inthe full-line position of FIG. 1, cuts off and rejects the passingportion of beam 24, whereas, in the dotted view position thereof itallows the passing portion of beam 24 to reach reference cell 33.

The chopper disc 28 is provided with a driving motor 35 which may bedriven at either a relatively low speed, of the order of 3750 RPM, or ata relatively high speed, of the order of 15,000 RPM, and which isaccurately stabilized by a synchronizing system 36 presently to bedescribed.

To compensate for chromatic aberration, namely, for varying index ofrefraction of the material of focussing lenses 114 and 27 with differentselected wavelengths of the excitation beam 24, provided by thepivotally adjustable monochromator grating 22, the position of lens 27is adjusted along its optical axis by a lens adjusting mechanism 37,presently to be described, which couples lens 27 to grating 22.

As shown in FIGS. 2 and 3, the chopper disc comprises a transparent bodyon which is provided a layer of suitable opaque material having planemirror surfaces on its opposite sides, shown respectively at 38 and 39,the disc being mounted so that it is inclined at an angle ofapproximately 45 to the axis of the focussing lens 27, as isdiagrammatically illustrated in FIG. 1. The mirror surface 38 faces thephotomultiplier tube 31. Said chopper disc is typically formed with fourequally spaced notches 40, each subtending an angle of 45, therebydefining four similarly spaced disc lobes 141, each likewise subtendingan angle of 45. Behind the chopper disc 28 are mounted a pair ofY-shaped fiber optic light pipe assemblies 41 and 42 having respectivestem portions 43 and 44 whose ends are located at common radialdistances from the axis of disc 28 and are spaced apart by an angle of45, being located to alternately transmit light to and receive lightfrom the reflective rear surface 39 of the chopper disc lobes 141, as isdiagrammatically shown in FIGS. 2 and 3.

The Y-shaped fiber optic light pipe assembly 41 has the light input arm45 and the light output arm 46. A lamp 47 is mounted in a position todeliver light to the end of input arm 45 and a photo diode 48 is mountedadjacent the end of arm 46 in a position to receive resultant lightpulses reflected from the chopper disc lobes 141. Thus, light from lamp47 travels through input arm 45 and stem 43 and is reflected as periodiclight pulses from the rear surfaces 39 of the lobes 141, travelling backthrough stem 43 and output arm 46 to the photo diode 48.

The Y-shaped fiber optic light pipe assembly 42 has the light input arm49 and the light output arm 50. A lamp 51 is mounted in a position todeliver light to the end of input arm 49 and a photo diode 52 is mountedadjacent the end of arm 50 in a position to receive resultant periodicreflected light pulses from the output arm 50.

The reflected periodic light pulses received by the respective photodiodes 48 and 52 are in alternating sequence, and generate similarlytime-spaced electrical signals which are essentially in phase with thetimeshared excitation beams 24,25 furnished to the sample cell 30 in thedual wavelength mode of operation shown in full-line view in FIG. 1, andalso in phase with the split beam components furnished respectively tothe reference cell 33 and the sample cell 30 in the split beam mode ofoperation.

The time-spaced electrical signal pulses from the photo diodes 48 and 52are delivered through respective amplifiers 53 and 54 to the inputs ofrespective pairs of monostable oscillators 55,56 and 57,58, therebyproviding respective trains of narrow pulses 59 in the output conductors61,62 of the monostable oscillators 55,56, and further trains of narrowpulses 60 in the output conductors 63,64 of the monostable oscillators57,58. The pulses 59 and 60 are time-spaced in the same manner asthe'light pulses reaching the photo diodes 48 and 52 and are thussynchronized with the time-shared dual monochromatic beams 24,25furnished to the sample cell30 in the dual wavelength mode of operation,or with the split beam components furnished respectively to thereference cell 33 and sample cell 30 in the split beam mode ofoperation.

The spectrophotometer is provided with a photomultiplier responsecircuit generally similar to that disclosed in the patent application ofGeorge W. Lowy et al., Ser. No. 291,046, filed Sept. 21, 1972, entitledDual Wavelength Photometer for Absorbance Difference Measurements, andin the patent application of R. Rodriguez, Ser. No. 297,279, filed Oct.13, 1972, entitled Dual Wavelength Photometer Response Circuit. lngthecircuit 65 the time-spaced signals generated in the photomultiplier tube31 are delivered through a current-to-voltage amplifier 66 to the poleof a two-position selector switch 67. The stationary contacts of theswitch 67 are connected respectively to the inputs of a conventionalvoltage amplifier 68 and a logarithmic amplifier 69. The outputs of theamplifiers 68 and 69 are connected to a conductor 70, which is in turnconnected through respective appropriately triggered electronic switchdevices 74 and 75 to respective track and hold integrating circuits 76and 77 which generate corresponding steady d.c. output voltagesrepresenting the time-spaced pulse signal voltages furnished toconductor 70. These two steady d.c. output voltages are appliedsimultaneously to the two input conductors 71 and 72 of a differentialamplifier 73. The output of amplifier 73, which substantially comprisesthe difference between the steady d.c. input signals from circuits 76and 77 is delivered to a suitable indicator 78, such as a recorder.

The electronic switch devices 74 and 75 may comprise field effecttransistors. The wire 61 is connected to the triggering electrode of thefield effect transistor 74 and the wire 64 is connected to thetriggering electrode of the field effect transistor 75. Thus, theelectronic switch devices are triggered by the pulses 59 and 60synchronously with the time-spaced light pulses reaching thephotomultiplier tube.

The operation of the track and hold integrating circuit 76 is asfollows: a pulse from conductor passes through a resistor 78 and fieldeffect transistor 74 to an input terminal 79 of an amplifier 80, and theamplified pulse appears at the output terminal 71 of amplifier 80. Acapacitor 81 connected between input terminal 79 and output terminal 71is charged to a d.c. voltage corresponding to the amplitude of theinput- 'output voltage drop. A discharge resistor 82 is connected acrosscapacitor 81 through field effect transistor 74 to allow the capacitorto adjust its charge with changing amplitudes of input pulses, thetriggering of the field effect transistor being synchronized with theinput pulses. Thus, a steady d.c. voltage appears at output terminal 71which is in accordance with the amplitude of the input pulse applied atterminal 79, this steady d.c. voltage being maintained until the nexttriggering of the field effect transistor 74, at which time its valuemay be changed because of a different amplitude of the next input pulse.The track and hold integrating circuit 77 operates in the same manner.

The time-spaced trains of narrow pulses 59 and 60 are also present inthe output conductors 62 and 63 of the monostable oscillators 56 and 57.Said conductors 62 and 63 are connected to the-inputs of a summingamplifier 83 which combines these pulses to derive a composite pulsetrain and counts the composite pulses, deriving an output voltage signalrepresenting the sum of the pulses per unit of time, said voltage signalappearing in the output conductor 84 of summing amplifier 83. Amplifier83 is of conventional construction and includes a well known pulsecounting circuit. The voltage signal in conductor 84 is thus inaccordance with the actual speed of rotation of the chopper disc 28,since the pulse frequency of the composite pulses is directlyproportional to the chopper disc speed.

The chopper disc driving motor 35 is energized from a d.c.-toa.c. 400cycle ac. inverter 85 which may be connected either directly to themotor or through a speed-reducing resistor 86, the resistor beingconnected in a branch circuit containing a shunting switch comprising aswitch pole 87 and associated contacts 96,97, forming part of a 2-pole,2-position speed control switch assembly 88. As shown diagrammaticallyin FIG. 3, said branch circuit is connected between the output conductor90 of inverter 85 and an input terminal of motor 35. The other pole 91of switch 88 is connected to an input wire 92 ofa differential amplifier93, and said pole 91 may be operated, simultaneously with pole 87, toconnect either a low reference voltage or a high reference voltage towire 92. The summer output conductor 84 is connected to the remaininginput terminal of the differential amplifier 93.

The output wire 94 of differential amplifier 93 is connected to the baseof an inverter control transistor 95 connected in a dc. supply conductorfor the inverter 85. The transistor 95, which operates as a switch, istherefore turned on responsive toa predetermined differential voltagesignal appearing at the output conductor 94 of the differentialamplifier 93, namely, when a corresponding difference in voltagedevelops between input wires 84 and 92. Such a difference in voltageoccurs when the chopper disc speed drops below a predetermined specificvalue, causing the inverter to be turned on by switch device 95 andcausing it to bring the motor 35 back-to the desired speed. When thedesired speed is attained, inverter 85 is turned off by the resultantturning off of the switch device 95, allowing the motor to coast untilthe above-described regulating action is repeated. As a result, thechopper disc 28 may be maintained with a high degree of accuracy ateither the selected relatively low speed, for example, 3750 RPM, or theselected-relatively high speed, for example, 15,000 RPM, in accordancewith the setting of the speed-selecting switch 88. I

It will therefore be seen that the system employed for synchronizing thegating of the field effect transistors 74 and 75 with the time-sharedlight beams reaching the photomultiplier tube 31 and the system employedfor regulating the speed of the chopper disc 28 are activated by commondriving means comprising common optical and optical transducercomponents, namely, the lamps 47,51, the fiber optic light pipeassemblies 41,42, the photo diodes 48,52, and their associatedamplifiers 53,54.

As shown in FIGS. 4 and 5, the monochromator grating 22 is mounted onone end of a supporting shaft 98 which is journalled in the oppositewalls 99 and 100 of a housing 101. Journalled in opposite walls 102 and103 of housing 101, transverse to shaft .98 and adjacent an end wall 104of housing 101 is a grating control shaft 105 which is provided withthreads 106 on which is engaged a travelling nut member 107, said nutmember having an end pin 108 which is slidably engaged in a slot 109formed in wall 104 parallel to shaft 106 and which prevents nut member107 from rotating in housing 101. Rigidly secured on shaft 98 is a levermember 110 having a first arm III extending toward shaft 105 and havinga second arm 112 extending at an obtuse angle to the first arm 111. Arm111 has a pin 113 secured thereto which engages beneath nut member 107,as viewed in FIG. 4.

A focussing lens assembly of the doublet type is employed, comprising afixed lens 114 mounted in a guide tube 115 secured in housing 101 and anadjustable lens 27 mounted in a carrier sleeve 116 slidably engaged inguide tube 115. Sleeve 116 is provided with a rack bar 117 fixedlysecured thereto which extends slidably through a longitudinal slot 118formed in tube 115. A pinion shaft 119 is journalled between walls 99and 100 parallel to shaft 98 and is provided with a pinion gear 120meshing with rack bar 117. Rigidly secured on shaft 119 is an arm 121formed with a cam edge 122 which is engaged by a pin 123 secured on theend portion of arm 1 12. Rack bar 117 is formed at its upper end, asviewed in FIG. 4, with a lug 124, and a biasing coil spring 125 connectslug 124 to the adjacent wall portion 126 of housing 101 biasing piniongear 120 and arm 121 in a clockwise direction, as viewed in said Figure.This urges cam edge 122 against pin 123 and thus biases lever member 110counterclockwise, as viewed in FIG. 4, urging pin 113 against thetravelling nut member 107. Thus, the grating 22 is coupled to movablelens element 27, because when control shaft 105 is rotated, nut member107 moves along the threads 106 and pin 113 follows this movement,causing lever member 110 to rotate and thereby adjust the angularposition of grating 22, while at the same time pin 123 moves along thecam edge 122 and causes arm I21 and pinion gear 120 to rotate andthereby move lens 27 element axially in tube [5. Cam edge 122 issuitably shaped and oriented to provide compensation for chromaticaberration of the focussing lens assembly.

As previously mentioned, the monochromator grat ing 23 is also pivotallymounted for angular adjustment so as to vary the wavelength of themonochromatic excitation beam 25. Suitably disengageable coupling means130 is provided for at times coupling grating 23 to grating 22.

The spectrophotometer 11 may be operated in various different modes,such as the following:

. Dual wavelength mode Split beam mode Dual wavelength scanning mode.Derivative mode Rapid kinetic mode Single beam mode.

In the dual wavelength mode, the monochromator grating 23 is set toprovide a reference wavelength beam 25, such as at an isosbesticwavelength, and the monochromator grating 22 is set ata different nearbywavelength, such as at a nearby absorption peak wavelength, and the gate34 is set in its closed position, with the gate 21 in its open position,as shown in full-line view in FIG. 1, so that the reference and samplebeams 24,25 are time-shared through the sample cell 30.

QUIAUN" In the split beam mode, the gate members 21 and 34 are set intheir dotted view positions of FIG. 1, and only the beam 19 is employed.The monochromator grating 22 is used to scan either the entire usablewavelength range or a relatively small increment of the range, and theworking beam 24 is time-shared through the reference cell 33 and thesample cell 30.

In the dual wavelength scanning mode, the gate members 21 and 34 are setin their full-line positions of FIG. 1 and monochromator grating 23 isset at a reference wavelength and monochromator grating 22 scans over awavelength increment, with the monochromatic reference and sample beams24,25 being time-shared through the single sample cell 30.

In the derivative mode, the gate members 21 and 34 are set in theirfull-line positions of FIG. 1 and the two monochromator gratings 23,22are locked together, by means of coupling mechanism 130, at slightlydifferent wavelengths, the two monochromatic beams 24,25 beingtime-shared through the single sample cell 30, and a derivative spectrumis obtained by scanning over a pre-selected wavelength range.

In the rapid kinetic mode, the optical chopper disc 28 is set to run, bymeans of switch 88, at the relatively high speed above-mentioned,namely, l5,000 RPM.

In the single beam mode, gate M is set in its dotted view position ofFIG. 1, providing the single beam 19, and the gate 34 may be set in itsfull-line position, wherein a single monochromatic excitation beam 24 isprovided through the sample cell 30.

. While a specific embodiment of an improved multiple-functionspectrophotometer has been disclosed in the foregoing description, itwill be understood that various modifications withinthe spirit of theinvention may occur to those skilled in the art. Therefore it isintended that no limitations be placed on the invention except asdefined by the scope of the appended claims.

What is claimed is: l. A spectrophotometer comprising two monochromatorsadapted to provide two respective monochro matic beams of radiation, aphotosensitive device, means to direct said two beams toward saidphotosensitive device substantially along a first optical path,timesharing chopper disc means in said first optical path and havingmeansto alternately direct said two monochromatic beams toward saidphotosensitive device along said first optical path, whereby saidmonochromatic beams can alternately pass through a transparent containerplaced in said first optical path between said chopper disc means andsaid photosensitive device to provide a dual wavelength mode ofoperation, and means to at times direct a portion of one of saidmonochromatic beams reaching said chopper disc means along a secondoptical path toward said photosensitive device, whereby said portion canpass through a transparent reference container placed in said secondoptical path, to provide a split beam mode of operation, and whereinsaid chopper disc means has a driving motor, said chopper disc meansincluding a notched disc defining spaced chopper segments, neans todetect variations in the speed of movement of said spaced choppersegments, and means to adjust the speed of said motor in accordance withsuch variations.

2. A spectrophotometer comprising two monochromators adapted to providetwo respective monochromatic beams of radiation, a photosensitive Idevice, means to direct said two beams toward said photosensitive devicesubstantially along a first optical path, timesharing chopper disc meansin said first optical path and having means to alternately direct saidtwo monochromatic beams toward said photosensitive device along saidfirst optical path, whereby said monochromatic beams can alternatelypass through a transparent sample container placed in said first opticalpath between said chopper disc means and said photosensitive device toprovide a dual wavelength mode of operation, and means to at timesdirect a portion of one of said monochromatic beams reaching saidchopper disc means along a second optical path toward saidphotosensitive device, whereby said portion can pass through atransparent reference container placed in said second optical path, toprovide a split beam mode of operation, and wherein at least one of themonochromators is provided with means to vary the wavelength of itsoutput monochromatic beam, said first-named directing means including anaxially movable focussing lens element, and means coupling said lenselement to said wavelength varying means to move the lens element so asto substantially compensate for chromatic aberration.

3. A spectrophotometer comprising two monochromators adapted to providetwo respective monochromatic beams of radiation, a photosensitivedevice, means to direct said two beams toward said photosensitive devicesubstantially along a first optical path, timesharing chopper disc meansin said first optical path and having means to alternately direct saidtwo monochromatic beams toward said photosensitive device along saidfirst optical path, whereby said monochromatic beams can alternatelypass through a transparent sample container placed in said first opticalpath between said chopper disc means and said photosensitive device toprovide a dual wavelength mode of operation, and means to at timesdirect a portion of one of said monochromatic beams reaching saidchopper disc means along a second optical path toward saidphotosensitive device, whereby said portion can pass through atransparent reference container placed in said second optical path toprovide a split beam mode of operation, and wherein said chopper discmeans has a driving motor, said chopper disc means including a notcheddisc defining chopper segments, means spaced angularly relative'to theaxis of the chopper disc to count said segments as they move therepast,means to add together the counts obtained by said spaced means, andmeans to adjust the speed of said motor in accordance with variations inthe total of the counts.

4. The spectrophotometer of claim 3, and wherein said photosensitivedevice is of a type generating electrical output pulse signals,respective integrating circuits connected to the output of saidphotosensitive device and being of a type generating steady d.c.voltages in accordance with the amplitudes of said pulse signals, andmeans to. gate said integrating circuits synchronously with the count ofsaid moving segments.

5. The spectrophotometer of claim 4, and means to compare said steadyd.c. voltages.

6. The spectrophotometer of claim 2, and wherein said one of themonochromators comprises a reflectance grating provided with rotarysupporting shaft means, and wherein said coupling means comprises alever member secured on said shaft means, rack and pinion meansdrivingly connected to said movable lens element, and interengaging camand follower means drivingly coupling said rack and pinion means to saidlever member.

7. The spectrophotometer of claim 6, and wherein said cam and followermeans comprises a projection on said lever member and a cam elementoperatively connected to said rack and pinion means and engaging saidprojection.

3. The spectrophotometer of claim 7, and means biasing said cam elementinto continuous engagement with said projection.

9. The spectrophotometer of claim 3, and means to operate said rotaryshaft means so as toangularly adjust said reflectance grating, saidoperating means comprising a rotary screw member and a travelling nutmember threadedly engaged on said rotary screw member and drivinglyengaging said lever member.

10. The spectrophotometer of claim 9, and wherein said lever member isprovided with a second projection and said travelling nut member islocated so that said biasing means urges said second projection towardcontinuous engagement with said travelling nut member.

11. The spectrophotometer of claim 3, and wherein said chopper discsegments have reflective surfaces and wherein said counting and addingmeans comprises spaced fiber optic systems located to receive lightreflections from said reflective surfaces and having respectivephotosensitive transducers generating pulse signals responsive to saidlight reflections, and means to derive a voltage representing the sum ofsaid pulse signals, and wherein said speed-adjusting means comprisesmeans to compare said sum with a reference voltage and derive adifference signal, and means to energize said motor responsive to saiddifference signal.

12. The spectrophotometer of claim 11, and wherein said photosensitivedevice is of a type generating electrical output pulse signals inaccordance with the intensity of the beams impinging thereon, respectiveintegrating circuits connected to the output of said photosensitivedevice, said integrating circuits being of a type generating steady d.c.voltages in accordance with the amplitudes of said last-named pulsesignals, and means to gate said integrating circuits synchronously withsaid light reflections.

13. The spectrophotometer of claim 112, and a differential amplifier,circuit means connecting the steady d.c. voltage outputs of saidintegrating circuits to the inputs of said differential amplifier, andindicating means connected to the output of said differential amplifler.

14. A spectrophotometer comprising monochromator means adapted toprovide a beam of monochromatic radiation, a photosensitive device,means to direct the monochromatic beam along an optical path leading tosaid photosensitive device, time-sharing chopper disc means in saidoptical path, said chopper disc means having a driving motor, saidchopper disc means including a notched disc defining spaced choppersegments, means spaced angulariy relative to the axis of the chopperdisc to count said segments as they move therepast, means to addtogether the counts ob tained by said spaced means, and means to adjustthe speed of said motor in accordance with variations in the total ofsaid counts.

15. The spectrophotometer of claim 14, and wherein said spaced countingmeans comprises spaced fiber optic systems located to sequentiallyreceive light re flections from said segments and having respectivephotosensitive transducers generating pulse signals responsive to saidlight reflections, said adding means comprising means to derive avoltage representing the sum of said pulse signals, and saidspeed-adjusting means comprising means to compare said sum voltage witha reference voltage and derive a difference signal and means to energizesaid motor responsive to said difference signal.

16. The spectrophotometer of claim 14, and wherein said photosensitivedevice is of a type generating electrical output pulse signals inaccordance with the intensity of the beams impinging thereon, respectiveintegrating circuits connected to the output of said photosensitivedevice, said integrating circuits being of a type generating steady d.c.voltages in accordance with the amplitudes of said last-named pulsesignals, and means to gate said integrating circuits synchronously withsaid counts.

17. The spectrophotometer of claim 15, and wherein said photosensitivedevice is of a type generating electrical output pulse signals inaccordance with the intensity of the beams impinging thereon, respectiveintegrating circuits connected to the output of said photosensitivedevice, said integrating circuits being of a type generating steady d.c.voltages in accordance with the amplitudes of said last-named pulsesignals, and means to gate said integrating circuits synchronously withsaid light reflections.

18. The spectrophotometer of claim 14, and wherein said monochromatormeans is provided with means to vary the wavelength of its outputmonochromatic beam, said directing means including an axially movablefocussing' lens element, and means coupling said lens element to saidwavelength-varying means to move the lens element so as to substantiallycompensate for chromatic aberration.

19. A spectrophotometer comprising monochromator means adapted toprovide an output monochromatic beam of radiation and provided withmeans to vary the wavelength of its output monochromatic beam, aphotosensitive device, means to direct the output monochromatic beamtoward said photosensitive device along an optical path leading to saidphotosensitive device, said directing means including an axially movablelens element, and means coupling said lens element to saidwavelength-varying means to move the lens element so as to substantiallycompensate for chromatic aberration.

1. A spectrophotometer comprising two monochromators adapted to providetwo respective monochromatic beams of radiation, a photosensitivedevice, means to direct said two beams toward said photosensitive devicesubstantially along a first optical path, time-sharing chopper discmeans in said first optical path and having means to alternately directsaid two monochromatic beams toward said photosensitive device alongsaid first optical path, whereby said monochromatic beams canalternately pass through a transparent container placed in said firstoptical path between said chopper disc means and said photosensitivedevice to provide a dual wavelength mode of operation, and means to attimes direct a portion of one of said monochromatic beams reaching saidchopper disc means along a second optical path toward saidphotosenSitive device, whereby said portion can pass through atransparent reference container placed in said second optical path, toprovide a split beam mode of operation, and wherein said chopper discmeans has a driving motor, said chopper disc means including a notcheddisc defining spaced chopper segments, neans to detect variations in thespeed of movement of said spaced chopper segments, and means to adjustthe speed of said motor in accordance with such variations.
 2. Aspectrophotometer comprising two monochromators adapted to provide tworespective monochromatic beams of radiation, a photosensitive device,means to direct said two beams toward said photosensitive devicesubstantially along a first optical path, time-sharing chopper discmeans in said first optical path and having means to alternately directsaid two monochromatic beams toward said photosensitive device alongsaid first optical path, whereby said monochromatic beams canalternately pass through a transparent sample container placed in saidfirst optical path between said chopper disc means and saidphotosensitive device to provide a dual wavelength mode of operation,and means to at times direct a portion of one of said monochromaticbeams reaching said chopper disc means along a second optical pathtoward said photosensitive device, whereby said portion can pass througha transparent reference container placed in said second optical path, toprovide a split beam mode of operation, and wherein at least one of themonochromators is provided with means to vary the wavelength of itsoutput monochromatic beam, said first-named directing means including anaxially movable focussing lens element, and means coupling said lenselement to said wavelength varying means to move the lens element so asto substantially compensate for chromatic aberration.
 3. Aspectrophotometer comprising two monochromators adapted to provide tworespective monochromatic beams of radiation, a photosensitive device,means to direct said two beams toward said photosensitive devicesubstantially along a first optical path, time-sharing chopper discmeans in said first optical path and having means to alternately directsaid two monochromatic beams toward said photosensitive device alongsaid first optical path, whereby said monochromatic beams canalternately pass through a transparent sample container placed in saidfirst optical path between said chopper disc means and saidphotosensitive device to provide a dual wavelength mode of operation,and means to at times direct a portion of one of said monochromaticbeams reaching said chopper disc means along a second optical pathtoward said photosensitive device, whereby said portion can pass througha transparent reference container placed in said second optical path toprovide a split beam mode of operation, and wherein said chopper discmeans has a driving motor, said chopper disc means including a notcheddisc defining chopper segments, means spaced angularly relative to theaxis of the chopper disc to count said segments as they move therepast,means to add together the counts obtained by said spaced meaNs, andmeans to adjust the speed of said motor in accordance with variations inthe total of the counts.
 4. The spectrophotometer of claim 3, andwherein said photosensitive device is of a type generating electricaloutput pulse signals, respective integrating circuits connected to theoutput of said photosensitive device and being of a type generatingsteady d.c. voltages in accordance with the amplitudes of said pulsesignals, and means to gate said integrating circuits synchronously withthe count of said moving segments.
 5. The spectrophotometer of claim 4,and means to compare said steady d.c. voltages.
 6. The spectrophotometerof claim 2, and wherein said one of the monochromators comprises areflectance grating provided with rotary supporting shaft means, andwherein said coupling means comprises a lever member secured on saidshaft means, rack and pinion means drivingly connected to said movablelens element, and interengaging cam and follower means drivinglycoupling said rack and pinion means to said lever member.
 7. Thespectrophotometer of claim 6, and wherein said cam and follower meanscomprises a projection on said lever member and a cam elementoperatively connected to said rack and pinion means and engaging saidprojection.
 8. The spectrophotometer of claim 7, and means biasing saidcam element into continuous engagement with said projection.
 9. Thespectrophotometer of claim 8, and means to operate said rotary shaftmeans so as to angularly adjust said reflectance grating, said operatingmeans comprising a rotary screw member and a travelling nut memberthreadedly engaged on said rotary screw member and drivingly engagingsaid lever member.
 10. The spectrophotometer of claim 9, and whereinsaid lever member is provided with a second projection and saidtravelling nut member is located so that said biasing means urges saidsecond projection toward continuous engagement with said travelling nutmember.
 11. The spectrophotometer of claim 3, and wherein said chopperdisc segments have reflective surfaces and wherein said counting andadding means comprises spaced fiber optic systems located to receivelight reflections from said reflective surfaces and having respectivephotosensitive transducers generating pulse signals responsive to saidlight reflections, and means to derive a voltage representing the sum ofsaid pulse signals, and wherein said speed-adjusting means comprisesmeans to compare said sum with a reference voltage and derive adifference signal, and means to energize said motor responsive to saiddifference signal.
 12. The spectrophotometer of claim 11, and whereinsaid photosensitive device is of a type generating electrical outputpulse signals in accordance with the intensity of the beams impingingthereon, respective integrating circuits connected to the output of saidphotosensitive device, said integrating circuits being of a typegenerating steady d.c. voltages in accordance with the amplitudes ofsaid last-named pulse signals, and means to gate said integratingcircuits synchronously with said light reflections.
 13. Thespectrophotometer of claim 12, and a differential amplifier, circuitmeans connecting the steady d.c. voltage outputs of said integratingcircuits to the inputs of said differential amplifier, and indicatingmeans connected to the output of said differential amplifier.
 14. Aspectrophotometer comprising monochromator means adapted to provide abeam of monochromatic radiation, a photosensitive device, means todirect the monochromatic beam along an optical path leading to saidphotosensitive device, time-sharing chopper disc means in said opticalpath, said chopper disc means having a driving motor, said chopper discmeans including a notched disc defining spaced chopper segments, meansspaced angularly relative to the axis of the chopper disc to count saidsegments as they move therepast, means to add together the countsobtained by said spaced means, and means to adjust the speed oF saidmotor in accordance with variations in the total of said counts.
 15. Thespectrophotometer of claim 14, and wherein said spaced counting meanscomprises spaced fiber optic systems located to sequentially receivelight reflections from said segments and having respectivephotosensitive transducers generating pulse signals responsive to saidlight reflections, said adding means comprising means to derive avoltage representing the sum of said pulse signals, and saidspeed-adjusting means comprising means to compare said sum voltage witha reference voltage and derive a difference signal and means to energizesaid motor responsive to said difference signal.
 16. Thespectrophotometer of claim 14, and wherein said photosensitive device isof a type generating electrical output pulse signals in accordance withthe intensity of the beams impinging thereon, respective integratingcircuits connected to the output of said photosensitive device, saidintegrating circuits being of a type generating steady d.c. voltages inaccordance with the amplitudes of said last-named pulse signals, andmeans to gate said integrating circuits synchronously with said counts.17. The spectrophotometer of claim 15, and wherein said photosensitivedevice is of a type generating electrical output pulse signals inaccordance with the intensity of the beams impinging thereon, respectiveintegrating circuits connected to the output of said photosensitivedevice, said integrating circuits being of a type generating steady d.c.voltages in accordance with the amplitudes of said last-named pulsesignals, and means to gate said integrating circuits synchronously withsaid light reflections.
 18. The spectrophotometer of claim 14, andwherein said monochromator means is provided with means to vary thewavelength of its output monochromatic beam, said directing meansincluding an axially movable focussing lens element, and means couplingsaid lens element to said wavelength-varying means to move the lenselement so as to substantially compensate for chromatic aberration. 19.A spectrophotometer comprising monochromator means adapted to provide anoutput monochromatic beam of radiation and provided with means to varythe wavelength of its output monochromatic beam, a photosensitivedevice, means to direct the output monochromatic beam toward saidphotosensitive device along an optical path leading to saidphotosensitive device, said directing means including an axially movablelens element, and means coupling said lens element to saidwavelength-varying means to move the lens element so as to substantiallycompensate for chromatic aberration.